1. Field of the Invention
The present invention relates to a silicon nitride sintered material and to a production process thereof. More particularly, the present invention relates to a silicon nitride sintered material which has a high thermal expansion coefficient, which maintains excellent insulating property, and which has high heat resistance so as to be suitable for use as an insulating material, such as a base material for ceramic glow plugs, as well as to a production process thereof.
2. Description of the Related Art
Conventionally, silicon nitride sintered material, having excellent mechanical characteristics, heat resistance, and anti-corrosion properties, has been employed as an insulating material for use in ceramic heaters in which a resistance heater is embedded, or as an insulating material in similar products. In this case, the silicon nitride sintered material must have a high insulating property. When silicon nitride sintered material is used as an insulating material, a problem arises in that since tungsten, tungsten carbide, molybdenum silicide, or a similar substance, which is generally employed as a resistance heater, has a thermal expansion coefficient higher than that of silicon nitride, cracks are generated in the insulating material as a result of the difference in thermal expansion coefficient during application of heat or generation of heat. In order to prevent the generation of cracks, the thermal expansion coefficient of the insulating material must be substantially as high as that of the resistance heater. Therefore, when the silicon nitride sintered material is used as an insulating material for ceramic heaters, etc., the sintered material must have a high insulating property and a high thermal expansion coefficient.
In order to increase the thermal expansion coefficient of the insulating material, conventionally, particles of rare earth element compounds such as oxides or metallic compounds such as carbides, nitrides, and suicides, etc., having a thermal expansion coefficient higher than that of silicon nitride have been incorporated and dispersed, as a high thermal expansion coefficient compound, into raw material powder of the silicon nitride sintered material. Typically, such a high thermal expansion coefficient compound is incorporated into silicon nitride sintered material in an amount of a few % to about 30% by volume.
The aforementioned rare earth element compound or metallic compound has a high thermal expansion coefficient, but its high conductivity raises a problem of lowering the insulating property of the sintered material upon incorporation of such a compound into the sintered material. Particularly when a large amount of a high thermal expansion coefficient compound is incorporated and dispersed in order to suppress a decrease in sinterability and strength, the compound tends to be present continuously in grain boundaries of sintered silicon nitride, to thereby greatly deteriorate the insulating property of the sintered material. In order to suppress such deterioration of insulating property, various studies have been carried out on the compositions and particle sizes of silicon nitride raw material and sintering aids. However, obtaining silicon nitride sintered material having a high thermal expansion coefficient while maintaining its insulating property remains difficult.